Whipstocks are devices that have long been in use to deflect a mill system through casing to create an opening known as a window. Whipstocks tend to be very long so that an inclined face on them can gradually nudge a mill system into the casing wall to start the window and to further guide the milling system until it makes an exit though the window. As a result the window shape is long and thin and narrows at the upper and lower ends. The slope on the whipstock guide surface is generally a small angle or series of angles.
Window milling with whipstocks has several unique issues that can affect the performance of the milling equipment. One concern has been when the center of the mill comes even with the casing wall and there is a tendency for the mill to bog down in that position so some designs have featured a steeply inclined surface on the whipstock guide face to engage the mill just as its center is in line with the casing wall. This kick-out surface occurs part way down the guide surface to address one specific problem.
Other approaches to whipstock design concern themselves with avoiding the milling of the whipstock at the onset of milling and configuring the contact angle between the mill and the whipstock guide surface at its upper end to be closer to parallel to decrease contact stress of the mill on the whipstock ramp at the top. The idea was to increase contact stress against the casing so that the casing would be milled rather than the top of the whipstock as the window is initiated.
There have been other approaches to insure getting a long enough window which have mostly been dealt with by making the ramp on the whipstock longer than the needed window length. The thinking here was that the longer ramp was insurance that the window length would not fall below a minimum desired dimension. In these designs, it was anticipated that the window mill will completely leave the ramp well before its lower end and this would insure the window was long enough and that the mill would safely enter the lateral. Even if this concept worked to give a window of a desired length, it created subsequent problems when downhole equipment needed to get through the window. FIG. 1 illustrates the problem. The whipstock face 10 extends downhole below the bottom of the window 12 in wellbore 14. In subsequent trips to get into lateral 16 and complete drilling and completion of it, a pocket trap 18 is created. If the later runs had mills or bits with blades, the blades would hang in the pocket 18 and the equipment would stall. The rig crew would sense this and have to stop rotation and pick up and then resume rotation and hope that on a subsequent attempt they would not direct a blade into the very same pocket trap. On the other hand if the equipment run subsequently had peripheral soft components like rubber seals or packer seals, the edge 20 of casing 22 acted as a razor sharp surface that could easily shred the softer components and render them unserviceable.
FIG. 2 shows another problem with whipstocks in the past. Here the mill fails to make an exit at the ramp end and simply continues to mill the whipstock base 24 and an anchor 26 below as indicated by the dashed line 28.
Over the years different ideas have been tried in whipstock designs but these problems have persisted. Several designs have tried gradual slopes and arcuate guide surfaces over the length of the whipstock all in the name of better mill guidance but none of these designs have eliminated the conditions depicted in FIGS. 1 and 2. Some examples of such designs are U.S. Pat. Nos.: 4,420,049; 3,116,799; 6,401,821; 2,699,920; 6,105,675; and 6,209,645. The last patent actually puts a radial surface 300 for the mill to catch on to hopefully define the lower end of the window.
One attempt to solve the above described problems has been to include an end surface on the guide face of the whipstock that is a sharper angle than the long gradual guide surface that is normally in the range of about 3 degrees. In soft formations the increase in angle at the ramp bottom puts a greater lateral load on the mill to help keep it against the whipstock surface as opposed to kicking out too early. In the hard formations the steeper end face directs the mill away from the whipstock to try to avoid having it continue down misdirecting the lateral and potentially damaging the whipstock or underlying anchor. With this approach it was hoped to eliminate the problems described above and left unresolved by the prior art but such was not the case.
The present invention addresses the needs at the start and conclusion of the window milling process. It features a rather large radius surface initially below the lug holding the window mill for run in. After some progress milling the window the ramp transitions to a greater angle to increase the lateral force on the mill and ensure its proper exit within the length of the ramp.
These and other features of the present invention will be more readily appreciated by those skilled in the art from a review of the description of the preferred embodiment and the claims that appear below.